1. Field of the Invention
The invention generally relates to a lifting device, and more particularly, to a lifting device enabling an object to stop at any position.
2. Description of Related Art
US Patent Publication No. U.S. Pat. No. 8,286,927 discloses a lifting device, which uses a plurality of cables to respectively connect between a pulley, a cam and a spring so as to perform an lifting adjustment of a supporting frame, and achieves a torque equilibrium through forces of the supporting frame and its installation equipment, or a gravity and a force of the spring, so as to enable the equipment can stay at any position. In addition, during a process of lifting adjustment for the supporting frame, the cam can be rotated for winding or releasing the cables on the cam, so that the supporting frame can have an adjustable stroke. However, as being limited by the shape of the cam, only the prior ¾ circle of the cam that can produce an effective torque equilibrium (as shown in FIG. 2 of U.S. Pat. No. 8,286,927), so that an effect of staying at any position can be achieved. If forcing the cam to rotate more than ¾ circle, then the effect of staying at any position would unable to be achieved; and if the rotation of the cam is being limited within approximately ¾ circle, then a length of the cable winding on the cam would be limited, and thus the adjustment stroke of the supporting frame would be reduced.